Make an object's visibility controlled by another object or an empty
If you only plan to use a circular mask you can use the Object option of the Cycles Texture Coordinate node to use any independent object as mapping origin for any texture.

This will allow using that object's position and transforms to control other material or texture through its position relative to the objects said material is applied to.

To do so just create a circular mask (you can do this easily with a Gradient texture set to Circular) and use it as mask to mix between two shaders. In your case mix whatever shader you liek to floor to be with a Transparent one to make it invisible outside a certain radius.

Use a Texture Coordinate probably set to Object coordinates and use the Object box to pick the desired empty - or sphere - to drive its positioning or scale. You can adjust the radius of the opaque part by scaling the empty.



You can achieve other "mask shapes" easily as long as you can recreate them procedurally with the available nodes. Otherwise you will probably have to use an image based texture for that. You can also try Dynamic Paint if you need the mask shape to actually follow some random object shape intersecting with the ground plane.